The present invention relates to a method of and an apparatus for growing a layer on a semiconductor wafer from vapor phase, and more particularly to a method of and an apparatus for growing a uniform layer on a multiplicity of semiconductor wafers each having a large diameter, from vapor phase.
In order to perform the vapor phase growth, as is well known, semiconductor wafers each acting as a substrate are placed in a reaction vessel, and a raw material gas is introduced into the reaction vessel while keeping the semiconductor wafers at an elevated temperature, to grow thin layers such as a thin monocrystalline silicon layer, a thin polycrystalline silicon layer, a thin silicon oxide layer and a thin silicon nitride layer, on the surface of each semiconductor wafer. The above vapor phase growth has been widely used for fabricating semiconductor devices such as a large scale integration circuit (LSI). An apparatus for vapor phase growth is required to satisfy the following conditions. (1) The thickness of thin layers grown on semiconductor wafers is uniform throughout the surface of each wafer, and among the wafers. (2) Crystal defects due to thermal stress or dusts are not introduced in semiconductor wafers themselves and in the grown layers. The vapor phase growth necessitates a complicated operation, requires a long processing time, and can treat only a small number of semiconductor wafers at a time. Accordingly, the vapor phase growth is comparatively high in process cost, as compared with other processes for growing a thin layer on semiconductor wafers. Thus, the apparatus for vapor phase growth is further required to satisfy the following conditions. (3) A large number of wafers can be treated at a time. (4) The apparatus has to be excellent in safety, since an inflammable, toxic gas is introduced into a high-temperature reaction vessel to form a thin layer on each wafer.
An apparatus for vapor phase epitaxy, that is, an apparatus for epitaxially growing a monocrystalline silicon layer on monocrystalline silicon wafers is strongly required to satisfy the above conditions (1) to (4), since the monocrystalline silicon layer is grown at elevated temperatures higher than 1,000.degree. C., and the electrical characteristics of a semiconductor device using the grown silicon layer are determined dominantly by the quality of the grown layer. The apparatus for vapor phase epitaxy has been improved, that is, a horizontal type furnace has been replaced by a vertical type furnace, and the vertical type furnace has been replaced by a barrel type furnace. Thus, apparatuses for vapor phase epitaxy capable of satisfying the conditions (1) to (4) are now available on the market. In recent years, there is a tendency that a semiconductor wafer becomes large in diameter, in order to increase the number of pellets taken out of the wafer, thereby reducing the manufacturing cost of a semiconductor device. Accordingly, it has become impossible for conventional apparatuses for vapor phase epitaxy to satisfy the condition (1) fully. Further, in the conventional apparatuses, a plurality of wafers are placed in a furnace in the state that the wafers are arranged in the same plane. Accordingly, it is difficult for the conventional apparatuses to satisfy the condition (3). Further, in order to treat a large number of wafers at a time, it is necessary to make the conventional apparatuses large in scale. In order to make large the scale of the conventional apparatuses, problems with respect to heat and difficulties in making a large-sized, high-purity susceptor have to be solved.
In order to solve the problems of the conventional apparatus for vapor phase epitaxy provided with the vertical type furnace or barrel type furnace, it has been tried to use a hot-wall type apparatus for vapor phase growth (hereinafter referred to as "A-type apparatus") which is disclosed in a Japanese patent examined publication No. 52-11198, for epitaxially growing a semiconductor layer. In the A-type apparatus, a plurality of wafers are arranged in a horizontally placed cylindrical reaction vessel which is disposed in a cylindrical, resistance-heating furnace, so that the principal surface of each wafer is perpendicular to the longitudinal axis of the cylindrical reaction vessel, to increase the number of wafers which are treated at a time. However, this apparatus has the following drawbacks. That is, a raw material gas is introduced into the reaction vessel at one end thereof, and is exhausted from the reaction vessel at the other end thereof. Accordingly, a layer grown on a wafer which exists on the upstream side, is different in thickness from a layer grown on another wafer which exists on the downstream side. Further, the raw material gas flows through a space between adjacent wafers nonuniformly, and hence the thickness of a layer grown on a wafer is not uniform. In order to eliminate the above drawbacks, a layer has been grown on each wafer under reduced pressure. In a case where wafers each having a large diameter are used, however, it is still very difficult to make uniform the thicknesses of grown layers on a plurality of wafers from practical point of view.
The hot-wall type apparatus (that is, the A-type apparatus) has a further drawback that the wall of the reaction vessel is heated as can be seen from the term "hot-wall", and thus a layer is also grown on the wall of the reaction vessel from the raw material gas. When wafers are loaded into and unloaded from the reaction vessel, or the temperature of the wafers is raised and lowered, the layer deposited on the wall of the reaction vessel may peel off, and becomes dust, which causes defects in the grown layers. Therefore, it is necessary to remove the deposit on the reaction vessel, e.g. by utilizing the reverse reaction of growth. The reaction vessel, however, may be reacted with the deposit to disable the perfect removal. Also, it makes the total process time longer.
The A-type apparatus has another drawback that a resistance-heating furnace with large heat capacity is used, and hence it takes a long time to lower the temperature of the inside of the furnace. In order to solve the above drawback, wafers are loaded into and unloaded from the reaction vessel while keeping the reaction vessel at an elevated temperature. In such a method, however, when large wafers each having a diameter of more than 5 in. (namely, about 12.5 cm) are used, the temperature of a wafer is not uniform at the surface thereof, and thus lattice defects due to thermal stress may easily be generated. In order to solve this problem, the wafers are introduced into and taken out of the reaction vessel, after the temperature of the reaction vessel has been lowered to about 800.degree. C. In this case, it takes several hours to lower the temperature of the reaction vessel, and hence the processing time is greatly increased.
The A-type apparatus has the drawback with respect to safety, that is, an additional drawback that the reaction vessel may be squashed. The wall of the reaction vessel is heated to a temperature higher than 1,000.degree. C., the inside of the reaction vessel is kept at a reduced pressure, and the wall of the reaction vessel is corroded with a layer grown on the wall. Accordingly, there is a danger of the reaction vessel being squashed. Specifically, in a case where the diameter of the reaction vessel is made large to be able to include large wafers, the danger of being squashed is a very serious problem.
As has been explained in the above, it seems impossible that the A-type apparatus capable of treating a multiplicity of wafers satisfies the above-mentioned conditions (1) to (4) fully. Specifically, a hot-wall type apparatus for epitaxially growing a semiconductor layer from vapor phase has not yet been put to the practical use.
In order to eliminate the drawbacks of the A-type apparatus, another kind of apparatus for vapor phase growth (hereinafter referred to as "B-type apparatus") has been proposed in a Japanese patent application (unexamined publication No. 59-50093).
In the B-type apparatus, a heater is placed in a reaction vessel, to avoid squashing of the reaction vessel due to the heating thereof. Further, in this apparatus, gas inlets are provided in the vicinity of wafers so that a raw material gas flows among the wafers, thereby supplying the raw material gas to the principal surface of each wafer uniformly. However, the outlet for waste gas is provided at an end of the reaction vessel in a direction perpendicular to the principal surface of each wafer, and thus a state that the gas flow (in particular, the exhaust gas flow) are the same at all the wafers, is not achieved. Accordingly, the B-type apparatus cannot solve the problem that a layer grown on a wafer which exists on the upstream side, is different in thickness from a layer grown on another wafer which exists on the downstream side.
In order to improve the nonuniformity of the thickness of grown layer in the gas flow direction, a different kind of apparatus for vapor phase growth (hereinafter referred to as "C-type apparatus") has been proposed in a Japanese patent application (unexamined publication No. 59-59878). In this apparatus, a gas supply nozzle is disposed so that a raw material gas flows among wafers, as in the B-type apparatus, but the outlet for waste gas is provided in the direction of the diameter of wafers, to prevent waste gas having flowed along the wafer from reaching other wafers, thereby making the difference in thickness between a layer grown on the wafer and a layer grown on another wafer as small as possible. In this apparatus, however, the thickness of a layer grown on a wafer becomes nonuniform at the surface thereof, when the diameter of the wafer is large. In more detail, in a method of supplying a raw material gas from the small opening of a nozzle to large scale wafers and discharging waste gas on the side opposite to the nozzle, it is difficult to supply the raw material gas to the whole area of the surface of the wafer uniformly, and hence a layer is grown on the wafer only along the gas flow path. Further, the C-type apparatus has a drawback that the growth of a layer on a wafer is encouraged in the vicinity of the opening of the nozzle. This drawback is very serious in an epitaxial growth process in which the reaction temperature is high, and the supply of a raw material gas to wafers (gas phase mass transfer to the growing surface) is a rate-determining step.
It can be readily thought that in the B-type apparatus having a similar structure to the C-type apparatus, also, the thickness of a grown layer becomes nonuniform at the surface of each wafer, when the diameter of wafers is large.
As has been explained in the foregoing, an apparatus for vapor phase growth has not yet been developed which can grow a uniform semiconductor or other layer having a high crystalline quality, on each of a large number of wafers epitaxially and safely.